Optical Holes Produce Better Memories

PHILADELPHIA — With today's technology, compact discs hold about 1 bit of information per square micron. Associate professor of physics Zameer Hasan and colleagues at Temple University have devised a way to increase this storage capacity by three orders of magnitude. The researchers believe that, in principle, 1 million bits of information per square micron eventually will be feasible, which would make it possible to fit the entire contents of the Library of Congress on just a few discs.

At Temple University, (from left) physicist Zameer Hasan and postdoctoral research fellows Levent Biyikli and Jhisook Park are investigating the optical properties of various materials to increase the storage capacity of compact discs.
Major challenges remain before this high storage density can be obtained. Besides finding the right optical material, developers must come up with faster storage and readout times, as well as nondestructive storage techniques. With a technique called photon-gated hole burning, alkaline earth sulfides doped with rare-earth ions can meet most of these challenges, the Temple researchers have found.

Photon-gated hole burning is a variation on spectral hole burning. Memories based on this technique use frequency-selective storage for multiple bits of information at the same location, thereby increasing storage density. With the materials that the Temple researchers are exploring, photon-gated hole burning produces more holes (up to 1000 per line); is permanent and efficient; and can use commercially available tunable red and IR lasers.

MgS:Eu2+ has produced the most promising system. Narrow holes can be burned efficiently at low power levels, the write beam produces little or no heating of the material and hole density can be increased by embedding the material in a polymer. This material also provides erasure-free readout, fast burning time (nanoseconds or less), an operating temperature above liquid helium (about 10 K), stability against temperature surges as high as 150 K and fast photo-erasure for multiple writes.

Increasing the operating temperature has been difficult, but it is critical for minimizing size and cost. The researchers have been able to increase the temperature to that of liquid nitrogen (77 K), but this has come at the expense of the storage density. Electron-phonon coupling -- the coupling of electrons to the lattice vibration of solids -- causes the low hole density at higher temperatures because the electron's energy is blurred rather than sharply defined.

"We are looking at several ways to eliminate or at least reduce the effect of electron-phonon coupling," Hasan said. "It will be a major breakthrough, but it is difficult to predict how long this will take."